No, it is not a practical security measure. I'm not going to speculate on the specific case you mentioned as I don't have all of the facts, but there are a couple of more generally relevant points.
First: Attackers do not determine which attacks might work based on the desktop chrome. They're going to figure out what the OS actually is, by any number of mechanisms. In addition, if someone were to have such a wacky idea, it would make far more sense to to attempt to pretend that a machine running an older, more vulnerable OS was newer, rather than the reverse described. Attempting to disguise Windows 7 machines as Windows 2000 by way of a desktop background would be not only useless, but uniquely and bizarrely useless.
Second: Withholding information about an OS from attackers is generally recognized to be a good practice, but of very minor importance. You'll see it regularly in pen-test results of web applications, for instance. There will be findings, deemed to be of low importance, suggesting the removal of headers identifying the web server software, or OS, or version, or the web application framework. They're findings because they can deter the very laziest or incompetent of attackers, but nobody would suggest that they would thwart a serious adversary from affecting a compromise, or even figuring out what the underlying technology really is.
So no, the case you described is most certainly not a security measure.